Why does Dickens attribute Wemmick with a very different behavior in his personal and professional life, a behavior that he follows as a work...

An interesting character, Wemmick figures into the motif
of doubles in Great Expectations. While Compeyson and Magwitch are
two convicts, Mrs. Joe and Miss Havisham two invalids, Biddy and Estella two women Pip
loves, Magwitch and Miss Havisham two benefactors, and two who wish to mold children to
their purposes, Wemmick is a double unto himself who acts as a transition father for Pip
in his search for paternal authority.  True to fact in Mr. Jaggers's office and true to
emotion at Walworth, Wemmick can only retain his integrity by closing off the squallid
world of London and Newgate Prison at his miniature castle.  Thus, he also figures into
the prison theme that permeates Dickens's novel.

Although
Pip, the penitential character who wrestles with guilt throughout the novel, feels
contaminated after having visited Newgate Prison with Wemmick, Mr. Jaggers's clerk walks
among the prisoners as one traverses a garden, bending and checking one, then another. 
Ironically, it is in the prison that Wemmick displays some of the kindness reserved for
Walworth, perhaps because they are removed from society.  He talks with the condemned
men, yet he realizes that they are doomed, and Pip narrates that he looks about him as
though wondering who will replace them: 

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With that he looked back, and nodded at his dead
plant, and then cast his eyes about him in walking out of the yard, as if he were
considering what other pot would go best in its
place.

Probably more than any
other character in the novel, Wemmick understands what a prison society is, a prison in
which people are misjudged and even convicted unjustly as is the falsely
convicted Colonel, whom Wemmick visits at Newgate.  Indeed, life in Victorian
society often cost people a fine, if not a sentence.  Because he perceives the
corruption of society, Wemmick wisely distances himself from the sordid world in which
he must work, keeping his "post office" mouth always the same.  For, it is only at
Walworth, and sometimes with Pip, that he can let down his guard against the prison that
is society.